chore(qso): drop the redundant SlotsSince free function

internal/qso/qso.go

@@ -2017,10 +2017,6 @@ type SlotRow struct {
 // SlotsSince lists the slots of every contact made since the given instant —
 // the contest watchlist's "worked today", where the day boundary lives in this
 // query's bound and nowhere else.
-func SlotsSince(r *Repo, ctx context.Context, since time.Time) ([]SlotRow, error) {
-	return r.SlotsSince(ctx, since)
-}
-
 func (r *Repo) SlotsSince(ctx context.Context, since time.Time) ([]SlotRow, error) {
 	rows, err := r.db.QueryContext(ctx,
 		"SELECT callsign, band, mode FROM qso WHERE qso_date >= ?",